Help with size of copper tubing and compression fitting

LTWFITTING 3/16 in. O.D. Brass Compression Coupling Fitting (10-Pack) HF62310 - The Home Depot

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What size of compression fitting do I need?

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3/8 OD Compression Copper Tube Union Straight Joiner Fitting Air Gas Water

Brass Compression Union, 1/4 – AGS Company Automotive Solutions

Brass construction Moody Union Fits 2-inch iron pipes. 3-inch body length, with coupling washers and nuts Use for pipe repair Install over pipe, or

Brass Moody Compression Coupling with 2-Inch Iron Pipe Size

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Putting my first loop together Are compression fittings THIS hard to put onto soft tubing?

61CL-10 Dixon Valve Brass Compression Fitting - Long Nut - 5/8 Tube S — HoseWarehouse